A cardiac arrhythmia is a condition in which the heart's normal rhythm is disrupted. There are many types of cardiac arrhythmias, including supraventricular arrhythmias that begin above the ventricles (such as premature atrial contractions, atrial flutter, accessory pathway tachycardias, atrial fibrillation, and AV nodal reentrant tachycardia), ventricular arrhythmias that begin in the lower chambers of the heart (such as premature ventricular contractions, ventricular tachycardia, ventricular fibrillation, and long QT syndrome), and bradyarrhythmias that involve slow heart rhythms and may arise from disease in the heart's conduction system.
Certain types of cardiac arrhythmias, including ventricular tachycardia and atrial fibrillation, may be treated by ablation (for example, radiofrequency (RF) ablation, cryoablation, ultrasound ablation, laser ablation, microwave ablation, and the like), either endocardially or epicardially. For example, atrial fibrillation (AF) is frequently treated with pulmonary vein ablation (also called pulmonary vein antrum isolation, or PVAI), a procedure that may involve inserting a mapping cryotreatment catheter through the left atrium of the patient's heart to the pulmonary vein (PV) ostium to map electrical impulses or potentials at the PV ostium before and/or after cryoablation. There is a depth within the PV at which electrical impulses are absent (such a location may be referred to as being “deep” within the PV), with the strength and/or prevalence of electrical impulses being greater closer to the PV ostium. The mapping catheter then may be inserted into the PV before ablation to act as an anchor to a cryoablation element and to support the cryoablation element during positioning at the left atrium/pulmonary vein (LA-PV) junction. Once the mapping catheter is properly seated within the PV, an ablation element (such as a cryoballoon or other ablation catheter configured to be advanced over a wire) is advanced over the mapping catheter until it is in contact with the ostium of the PV, within the left atrium. Proper contact between the cryoballoon and the PV ostium, which results in PV occlusion, may be confirmed using visualization techniques such as fluoroscopy.
Once the cryoballoon is in good position, the mapping catheter is slowly pulled back from deep within the PV to an area closer to the PV ostium. It is desirable to position the mapping catheter as close to the cryoballoon as possible, as the mapping catheter at this location may offer the best chance of contact with tissue that has electrical activity and thus detection and recordation of pulmonary vein potentials (PVPs). Recording these PVPs may provide insight as to the time-to-effect during onset of ablation. Additionally, mapping tissue far distal to the cryoballoon may not provide accurate data about the target isolation site and the effectiveness of cryotreatment and may pick up far field electrical signals that could interfere with the ability of the user to decipher or interpret the signals. Although it is desirable to collect this additional data, users are often forced to leave the mapping catheter deep within the PV during the cryotreatment procedure (for example, cryoablation). This is because, in some cases, retraction of the mapping catheter once the cryoballoon is in place reduces or eliminates the support provided to the cryoballoon by the mapping catheter, and the cryoballoon may slip out of place (that is, occlusion of the PV may be compromised). In those cases, the user must re-advance the mapping catheter to a more distal location within the vein and reposition the cryoballoon. Further, repositioning the cryoballoon typically involves reassessing PV occlusion, such as by the injection of a contrast medium from the cryoballon lumen (such as a guide wire lumen, within which the mapping catheter is slidably disposed) and imaging by fluoroscopy. The use of contrast medium and fluoroscopy not only exposes the patient and clinicians to radiation, but is sometimes poorly tolerated by some patients, including those with renal insufficiency.
During the cryotreatment procedure (for example, cryoablation), refrigerant circulating through the cryoballoon absorbs heat from surrounding tissue. As the tissue freezes, blood adjacent the treatment site may also freeze, creating an “ice ball” that temporarily adheres the cryoballoon to the tissue at the treatment site, a phenomenon called cryoadhesion. Once cryoadhesion occurs, retraction of the mapping catheter from within the PV has less of an effect on cryoballoon stability and could, in theory, be withdrawn and used to detect and record PVPs proximate the ablation site. However, within about ten seconds from commencement of the cryotreatment procedure, fluids within the guide wire lumen around the mapping catheter freeze, effectively locking the mapping catheter in place and preventing its axial movement. Although some currently known methods may involve retraction of the mapping catheter before the onset of freezing (that is, within the first approximately ten seconds), there are several drawbacks to this method. For example, cryoadhesion between the cryoballoon and the tissue may not yet have occurred, and movement of the mapping catheter without cryoadhesion will unseat the cryoballoon and require repositioning of the cryotreatment device.
It is desirable, therefore, to provide a system and device that allows for the axial movement of the mapping catheter during all stages of cryotreatment so that the mapping catheter may not only map LA-PV tissue before and after cryotreatment and anchor the cryoballoon against the PV ostium, but also allow for mapping of the PV tissue proximate the PV ostium during cryotreatment as well. Further, even when the mapping catheter is positionable proximate the cryoballoon, the local temperature may cause ice formation on one or more of the mapping electrodes of the mapping catheter, resulting in attenuation or loss of signal. Therefore, it is further desirable to provide a system and device that prevents or thaws ice formation on mapping catheter electrodes positioned proximate a cryoballoon during a cryotreatment procedure.